1. Field of the Invention
The present invention relates to an optical amplifier which amplifies a light signal inputted into one end of a rare-earth doped optical fiber by transforming an excitation light injected from the other end of the rare-earth doped optical fiber into an excitation energy.
2. Technical Background
FIG. 5 is a schematic diagram of a first example of a conventional optical amplifier. A rare-earth doped optical fiber 1 (shortened to optical fiber 1 hereinbelow) is produced by doping an optical fiber material with rare-earth elements, for amplifying an input optical signal inputted into one end 1a of the optical fiber 1. The other end 1b of the optical fiber 1 is optically connected to an optical coupler 2. The optical coupler 2 is for guiding the input optical signal from the one end 1a of the optical fiber 1 to the optical output terminal 3 as well as for injecting an excitation light generated by the excitation light source 4 into the optical fiber 1 into the other end 1b of the optical fiber 1.
The excitation light source 4 is driven by an excitation light source driving signal S.sub.D supplied from the excitation light source driving circuit 5, and generates an excitation light having a light energy of a constant light intensity. The excitation light generated by the excitation light source 4 is injected into the optical fiber 1 from the other end 1b of the optical fiber 1 through the optical coupler 2. The light energy supplied by the excitation light, injected into the optical fiber 1, is transformed into an excitation energy to be accumulated therein, and is utilized as an excitation energy for effecting optical amplification. The optical fiber 1 acts as an optical amplifier by transforming the accumulated excitation energy to an optical energy signal having the identical wavelength and phase characteristics to the optical signal inputted into the one end 1a of the optical fiber 1. The amplified optical signal is outputted from the optical output terminal 3 through the optical coupler 2.
The optical gain through light amplification within the optical fiber 1 is dependent on the amount of excitation energy accumulated in the interior of the optical fiber 1. Therefore, in this type of optical amplifiers, when an input optical signal such as the one shown in FIG. 6A is inputted into the one end 1a of the optical fiber 1, produces an output optical signal having a distorted waveform shape, such as the one shown in FIG. 6B.
The reason for this behavior of light signal outputted from the optical amplifier of the type shown in FIG. 5 will be explained in the following. First, when there is no input optical signal from the one end 1a of the optical fiber 1, the optical energy emitted from the excitation light source 4 and injected into the optical fiber 1 is not utilized to amplify any light signal even when it is transformed into excitation energy within the optical fiber 1. Therefore, the excitation energy becomes accumulated within the optical fiber 1. As explained above, because the degree of light amplification (optical gain) of optical fiber 1 is dependent on the amount of excitation energy accumulated, the optical gain increases as the amount of excitation energy accumulated within the optical fiber 1 increases.
As explained already, the optical energy supplied by the excitation light injected into the other end 1b of the optical fiber 1 is transformed into excitation energy to be utilized for light amplification within the optical fiber, however, a part of the excess excitation light is emitted from the one end 1a as a leakage light signal. Furthermore, a part of the excess excitation energy accumulated within the optical fiber 1 produces natural emission light in the interior of the optical fiber 1, and is discharged from the optical fiber 1.
The amount of excitation energy accumulated within the optical fiber 1 having no signal input from the one end 1a reaches a steady value determined by the balance among the light energy injected from the other end 1b, leakage light and natural emission light. Therefore, the optical gain of an optical fiber 1 having no input signal assumes a large steady state value.
When an optical signal is inputted into the one end 1a of the optical fiber 1 in the condition described above, because the capacity for light amplification of the optical fiber just prior to the optical signal input event is high, an output optical signal of a high intensity is outputted from the output optical signal from the optical output terminal 3, as shown in FIG. 7. The amount of excitation energy consumed by the light amplification process is larger than the light energy of the excitation light injected from the other end of the optical fiber 1, therefore, the excitation energy accumulated within the optical fiber 1 is consumed by the process of optical amplification, and the residual amount decreases, thereby leading to a lower light amplification of the input light.
The result is that, in spite of the fact that the input optical signal at the one end 1a of the optical fiber 1 has a constant light intensity, the output light intensity from the other end 1b of the optical fiber 1 decreases. Subsequently, a balance is reached between the consumption of excitation energy due to optical amplification and the supply of light energy from the excitation light source 4 is achieved, and a steady state is produced.
As explained above, the degree of light amplification of an optical fiber 1 excited with an excitation light of a constant intensity exhibits a transitory behavior with respect to a rectangular waveform as shown in FIG. 6A, leading to a distorted output waveform such as the one shown in FIG. 6B. The transient period of the output signal variation can last from several hundreds of micro-seconds to as high as several milli-seconds.
Therefore, to obtain an optical signal having no waveform distortion from an optical amplifier, it is necessary to maintain a steady optical gain within the optical fiber 1. A conventional optical amplifier having a provision for maintaining a steady light amplification in an optical fiber 1 will be explained with reference to FIG. 8. Those components in FIG. 8 which are the same as in FIG. 5 are referred to by the same reference numerals.
In FIG. 8, the one end 1a of the optical fiber 1 is optically connected to a optical input terminal 7 through a wavelength multiplexer 6. The wavelength multiplexer 6 performs the task of inputting an input optical signal (to be amplified), received from the optical input terminal 7, into the one end 1a of the optical fiber 1 as well as inputting the leakage light signal from the optical fiber 1 into a photodetector element 8. The optical coupler 2 guides the light signal outputted from the other end 1b of the optical fiber 1 to the optical output terminal 3, and inputs excitation light emitted from the excitation light source 9 having a monitor to the other end 1b of the optical fiber 1.
The excitation light emitted from the monitor-equipped excitation light source 9 is injected into the other end 1b of the optical fiber 1 through the optical coupler 2. The light energy supplied by the excitation light which is injected into the optical fiber 1 from the monitor-equipped excitation light source 9 is transformed into excitation energy within the optical fiber 1, and is accumulated in the interior of the optical fiber 1 to be utilized as excitation energy for light amplification. The optical fiber 1 performs the task of optical amplification by transforming the excitation energy accumulated within to an optical signal having identical wavelength and phase characteristics to the input optical signal inputted from the one end 1a of the optical fiber 1. The amplified optical signal is outputted from the optical output terminal 3 through the optical coupler 2.
The electrical signals required for controlling the optical gain of the optical fiber 1 are a monitor signal S.sub.M which is proportional to the light intensity of the excitation light emitted from the monitor-equipped excitation light source 9 and a leakage signal S.sub.L which is proportional to the light intensity of the leakage light signal emitting from the optical fiber 1. The division circuit 10 performs the division of leakage signal S.sub.L with the monitor signal S.sub.M. The output signal from the division circuit 10 is proportional to the optical gain of the optical fiber 1.
Therefore, by feeding back the output signal from the division circuit 10 to the monitor-equipped excitation light source 9, it is possible to maintain the degree of light amplification of the optical fiber 1 at a constant level. This is performed by driving the monitor-equipped excitation light source 9 with an excitation light source driving signal S.sub.D obtained by dividing a sum of an inverse output signal S.sub.F from the division circuit 10 and a standard signal S.sub.S generated from the standard electrical source circuit 11 so as to stabilize the optical gain of the optical fiber 1.
One of the problems in the conventional feedback type optical amplifier shown in FIG. 8 is that the degree of light amplification can be stabilized when the light intensity of the optical signal inputted into the optical input terminal 7 is either constant or changing slowly.
However, when the light intensity of the input signal into the optical input terminal 7 increases quickly as illustrated in FIG. 6A, the feedback control alone as shown in FIG. 8 gives rise to a waveform distortion of the output light from the optical output terminal 8 as illustrated in FIG. 7. This is because the excitation energy within the optical fiber 1 is consumed by the amplification process of the input optical signal, and the feedback control by the control elements 8.about.10 and 12 are not able to respond in time to compensate the lowered optical gain caused by the rapid rise in input light intensity. In other words, there is a time delay in increasing the light energy supply to compensate the drop in the excitation energy in the optical fiber 1, thus leading to a temporary drop in the amplification gain in the optical fiber 1.
There are two reasons for the time delay in feedback control of the circuit design presented above. First, the transformation process of the excitation light into light energy is a time-dependent process, and second, the leakage light signal is dependent on the accumulated light energy in the optical fiber 1. Therefore, the waveform of the leakage signal S.sub.L is related to an integral of the light intensity of the input optical signal, and consequently the feedback signal S.sub.F can only change gradually. Therefore, there is a problem that the feedback control technique for a shot type waveform (e.g. rectangular) inevitably results in a distorted amplification gain.
Furthermore, so long as a circuit of the type illustrated in FIG. 8 is used for feedback control, it is not possible to resolve the time delay problem of amplification gain. This is because of the inherent problem of time-dependent process in transforming the light energy of the excitation light to excitation energy and in the delayed response in the leakage light signal. It follows that even if the components of the feedback control circuit, such as the photodetector elements 8, monitor-equipped excitation light source 9, division circuit 10 and the addition circuit 12 are changed to faster performing devices, the response time of the feedback control circuit cannot be expected to be improved significantly.